Telephone communication systems are continually developed to provide ever increasing convenience for users. In voice communication, users seek telephone equipment which provides effective "transparency", that is to say that the user of a telephone wishes to communicate with his or her counterpart in a manner that allows each person to become unaware of the telephone. An ideal voice-user telephone device should be able to transmit voice communication between two or more parties without either of the parties being encumbered with a need to hold a telephone instrument. Speakerphones provide this transparency, but at a cost which is prohibitive to many potential users.
A speakerphone in its simplest configuration can consist of a conventional handset with an ear to mouth arrangement and an additional microphone and speaker which permit a user to speak and hear at a distance of at least a few feet from the speakerphone set. Unfortunately, this simple speakerphone configuration, while relatively inexpensive, does not in may instances provide satisfactory results because of echoes which are created in speech by sound reflections from walls of rooms and offices. These echoes form in the speech created within the room by a user of the speakerphone and also in the speech transmitted to the room by the speakerphone. Left uncontrolled, these echoes make speakerphone communication virtually unintelligible.
The control of echoes has been accomplished in various ways in the prior art. The most common form of echo control is attained by operating the speakerphone in a half-duplex mode. In this technique, a sound-actuated switch is used to control transmission over the speakerphone to a single direction at any time, thus preventing a speaking party's echoes from being re-transmitted to that party. Control circuitry needed to achieve this half-duplex form of operation adds substantially to the cost of a speakerphone and in large measure is responsible for the fact that speakerphones are not in universal use in residences and businesses in spite of their desirability.
However, even in circumstances where half-duplex speakerphones can be cost justified, there remains in these units a rather undesirable lack of realism to the conversations which take place. If, while using a half-duplex speakerphone mode, a first party is speaking to a second party who starts to speak simultaneously, the words of the second party are not heard by the first party. Also, if background noise exists at the speakerphone location, the second party may not be heard at all because the control circuitry, which is usually sound actuated, may cause the half-duplex system to remain switched in the transmit mode.
These problems associated with half-duplex operation are correctable with control circuitry such as that described in U.S. Pat. No. 4,912,758 (Ygal Arbel). Through echo cancellation techniques, a speakerphone can be made to operate in a full-duplex mode. But in this mode the control circuitry is very expensive and speakerphones of the full-duplex type have been heretofore typically used only in elaborate conference room settings.
It is desirable to have speakerphone capability with half or full-duplex mode without the present high cost.